Polypropylene in its many and varied forms is a long establish staple of the polymer industry. Depending upon its form, it exhibits a number of desirable properties including toughness (as measured by any of a number of impact tests, e.g., notched Izod, dart drop, etc.), stiffness (as measured by any of a number of modulus tests e.g., Young's), clarity, chemical resistance and heat resistance. Often a particular combination of properties is desired that requires a balancing of various properties against one another (e.g., stiffness against toughness).
Crystalline polypropylene, typically a homopolymer, is used extensively in various moldings because it exhibits desirable mechanical (e.g., rigidity) and chemical resistance properties. For applications that require impact resistance (e.g., automobile parts, appliance facia, packaging, etc.), a copolymer of propylene and ethylene (P/E copolymer) and/or one or more α-olefins is used, or a blend of crystalline polypropylene with one or more polymers that exhibit good impact resistance, e.g., ethylene-propylene (EP) and/or ethylene-propylene-diene (EPDM) rubber. For applications that require toughness and/or heat resistance (e.g., films), preferably the polypropylene has a relatively low melt flow rate (MFR) or expressed alternatively, a relatively high weight average molecular weight (Mw). For applications that require good processing characteristics (e.g., fibers), preferably the polypropylene has a relatively narrow polydisperity or molecular weight distribution (MWD), e.g., less than 3.5.
One method of modifying the properties of polypropylene, either as a homopolymer or as a copolymer, is to modify its crystalline structure. The onset of crystallinity is known as nucleation, and this may occur randomly throughout the polymer matrix as the individual polymer molecules begin to align. Alternatively, nucleation may occur at the interface of a foreign impurity or an intentionally added nucleating agent. The proper use of nucleating agents can result not only in unique and desirable crystalline structures, but they can also promote the efficiency of a given process by shortening process times, initiating nucleation at higher temperatures and the like.